1. Field of the Invention
The present invention relates to a hinge mechanism for a platen cover of an image forming apparatus for holding an original document placed on a platen glass at a fixed position, wherein the platen cover is installed in a manner that allows an operator to open and close the platen cover when placing and removing the original document.
2. Description of the Related Art
A document reading section of an image forming apparatus, such as various types of copying machines and printers, is provided with a platen cover, which can be lowered onto and lifted up from a platen glass, for holding an original document placed on the platen glass at a fixed position and for keeping out stray light from an optical scanning system. Normally, this kind of platen cover is swingably supported with its pivot end fixed to a housing of the apparatus by means of hinges. Each hinge is usually associated with an elastic member for biasing the platen cover in its opening direction to ensure smooth opening and closing action of the platen cover.
A hinge mechanism employing a torsional compression coil spring as an elastic member is disclosed in Japanese Laid-open Patent Publication No. 2002-23431, for example. Also, a hinge mechanism employing a compression coil spring as an elastic member is disclosed in Japanese Laid-open Patent Publication No. 2001-57611, for example. As shown in FIGS. 7A and 7B, for instance, a hinge unit 52 supporting a platen cover 51 is normally inserted into and held in a cylindrical hinge holder 53 provided at one end of a housing of an image forming apparatus together with a compression coil spring 54 in such a manner that the platen cover 51 can be opened and closed by swinging it up and down about a pivot axis of the hinge unit 52.
In a case where the hinge unit 52 is accommodated in the hinge holder 53 as stated above, however, the compression coil spring 54 would occasionally produce violent vibrations inside the hinge holder 53 when the platen cover 51 is opened or closed. If such vibrations occur from time to time, the hinge holder 53, usually made of a rigid resin material, is apt to break after an extended period of use. This would result in such a problem that the platen cover 51 lies loose on the platen cover, causing displacement of an original, or stray light enters an optical scanning system. This kind of problem is assumed to occur for reasons explained below.
The conventional image forming apparatus is designed such that a hinge torque exerted by the compression coil spring 54 of the hinge unit 52 biasing the platen cover 51 in its opening direction remains at zero, or no load conditions, near the closed position of the platen cover 51 at and below a point of free fall (within a range of 0° to a few degrees above 10° in terms of the opening angle of the platen cover 51) as shown in FIG. 6, for example. With this arrangement, the compression coil spring 54 exerts no biasing force (hinge torque) on the platen cover 51 near its closed position, so that the platen cover 51 descends by its own weight down to the closed position with certainty.
This construction serves to prevent the platen cover 51 from being left unclosed. However, if the platen cover 51 is hastily closed for one reason or another, the compression coil spring 54, which suddenly turns from a loaded state to an unloaded state, violently vibrates inside the hinge holder 53 and collides with its internal wall. This would gradually lead to eventual breakage of the hinge holder 53 from inside.
More specifically, when the platen cover 51 is opened to a certain degree, a roller 57 fitted in and supported by a roller retainer 56, which is forced upward by a spring force directed vertically upward, comes into contact with a cam element 55 which is fixed to a pivot end of the platen cover 51 as shown in FIG. 7A, so that the roller 57 biases the cam element 55 upward. This biasing force of the roller 57 produces the aforementioned hinge torque that causes the platen cover 51 to swing in its opening direction, wherein the hinge torque is defined as the product of a vector pointing to the cam center (representing a force acting in the direction of a perpendicular P to a tangent line passing a point of contact between the cam element 55 and the roller 57) multiplied by the distance d from the point of contact to the point of support of the hinge unit 52.
When the platen cover 51 is closed, on the other hand, the perpendicular to the tangent line passing the point of contact between the cam element 55 and the roller 57 which is forced upward by the spring force directed vertically upward passes a pivot center 58 of the cam element 55 (provided above the hinge holder 53) that serves as a point of support of the platen cover 51 as shown in FIG. 7B, so that the hinge torque becomes zero. This means that the hinge torque biasing the platen cover 51 in its opening direction is zero in a state in which the compression coil spring 54 is slightly depressed (compressed).
Therefore, if the platen cover 51 is hastily closed from its open position, the compression coil spring 54 rapidly changes to the compressed state in which the hinge torque is zero. As a consequence, pressure balance becomes unstable and elastic force of the compression coil spring 54 causes itself to vibrate violently in the hinge holder 53 and collide with its internal wall, and this causes gradual breakage of the hinge holder 53, particularly from its lower portion.
Although not illustrated in detail, there is provided a clearance between the internal wall of the hinge holder 53 and the compression coil spring 54 to allow flexible compression and extension of the compression coil spring 54. For this reason, the compression coil spring 54 is likely to oscillate loose within the hinge holder 53, producing violent vibrations when the platen cover 51 is opened or closed.